The present invention relates in general to the field of high frequency power conversion. More particularly, the present invention relates to a high frequency switch mode type power supply which generates an ac-type waveform by rapidly switching on and off one or more dc voltage sources. Still more particularly, the present invention relates to a base drive circuit which is used to couple turn-on and turn-off signals to a power amplifying device such as a bipolar transistor utilized in the inverter section of a power supply.
Referring now to FIG. 1, those skilled in the prior art will realize that a bipolar transistor 10 may be used as an ON-OFF switch to energize the load 12 with a voltage source, VL. Thus, transistor 10 will be turned on and the load 12 energized whenever base current IB1 is forced to flow. In order to quickly de-energize the load 12, it is generally insufficient to set IB1 to zero since minority carriers stored in the base region of transistor 10 must recombine before collector current will cease to flow following the interruption of IB1. The resulting delay, referred to as storage time, is in general unacceptably long unless the charge stored in the base region of transistor 10 is removed by external means such as a reverse base drive current IB2. Current reversal can be accomplished by reversing the polarity of the bias voltage VBE. The use of reverse base-emitter voltage has the further advantages of decreasing the collector current fall time during turn off and allowing the transistor 10 to withstand higher collector-emitter voltages once it is in the OFF state.
In the design of switch mode power supplies, the object is to produce an ac-type waveform which will then be transformed down, rectified and filtered to produce a dc voltage which is typically used to power some logic circuitry. To generate the ac-type waveform, switch mode power supplies often utilize two power switches such that when one switch is turned on the other switch is turned off. One of the switches controls the application of a positive dc voltage to the load while the other switch controls the application of a negative dc voltage, with the result that by operating the switches in a push-pull manner an ac-type voltage is applied to the load.
Bipolar transistors are frequently used as the power switches because of their fast switching speed and ability to handle high voltages and currents. In order to rapidly turn on and off the bases of the bipolar transistors and thus be able to generate a high frequency ac-type voltage, a base drive circuit for coupling control signals into the base of the power switch (i.e., bipolar transistor) while at the same time achieving fast turn off of that power switch is required.
The design of base drive circuits for power inverters is well known in the prior art. Thus, for example, in "Base Drive Considerations in High Power Switching Transistors" by Dennis Roark, TRW Power Semiconductors Application Note No. 120, January, 1975, a circuit to provide transformer isolated base drive which is readily employable in a power inverter is described. When such a base drive circuit is used in an inverter, two of the base drive circuits are connected to the secondary of an isolation transformer, each to couple one of two switching signals from the primary side of the isolation transformer to one of two power switches respectively, the two power switches energizing one side of the load with either the common or V+ output of a high voltage dc power supply, respectively, the other side of the load being connected to a dc voltage of +V/2, thus applying an ac-type voltage to the load.
A first problem with the latter discussed prior art circuit occurs due to the less than perfect characteristics of practical coupling transformers, which can result in both power switching transistors conducting simultaneously under certain operating conditions. This simultaneous conduction, called cross conduction, is equivalent to a short circuit being placed across the high voltage supply and leads to degradation and eventual destruction of the power switching transistors. The operating conditions referred to are those where the power switching transistors are turned on for time durations which are short relative to the overall period of operation; viz, low duty cycle operation. The problem experienced with low duty cycle operation may in some cases be ovecome by utilizing an optimized coupling transformer. But even with an optimized coupling transformer, there would be a limit on how far the pulse width could be reduced before cross conduction problems would be experienced.
A second problem with the latter discussed prior art circuit occurs due to the inability to control both IB1 and IB2; viz, the user can pick either IB1 or IB2, but is normally stuck with the resulting value of IB2 or IB1, respectively. Although somewhat dependent on the transistor's characteristics, in general the greater the value of IB2, the faster the transistor will turn off; viz, the transistor's turn-off time is inversely proportional to IB2. The inability to control both IB1 and IB2 oftentimes presents a problem since the value of IB1 is often dictated by the application, thus giving the designer no choice as to the value of IB2. Thus, for example, in a typical switching power supply, the value of IB1 is fixed by the gain of the transistor which when multiplied by IB1 must be equal to or greater than the load current.
The cross-conduction problem of the prior art has been overcome by the invention described in pending U.S. patent application, Ser. No. 146,648, which is incorporated and made part of the present disclosure. It is a general object of the present invention to overcome both of the identified prior art problems by providing an improved circuit for coupling control signals into the base of a power switch.
It is a further object of the present invention to provide an improved base drive circuit which allows the user to select the values of both the forward base drive current IB1 and the reverse base drive current IB2.
It is another object of the present invention to provide an improved base drive circuit which allows the user to meet load current requirements while simultaneously allowing the desired storage time to be obtained.
It is a further object of the present invention to provide a circuit for coupling control signals into the base of a power switch and at the same time achieving fast turn off of the power switch.
It is another object of the present invention to provide an improved high frequency switch mode type power supply.
It is still another object of the present invention to provide an improved inverter circuit capable of operating with a very short duty cycle.
It is still a further object of the present invention to provide an improved transformer isolated base drive circuit which provides a polarity insensitive input impedance.
These and other objects, features and advantages of the present invention will become more apparent from the detailed description of the preferred embodiment when read in conjunction with the drawings.